This invention describes a novel process for producing anhydride-containing polymers for radiation sensitive compositions.
Typically polymers based on anhydride monomers are produced by free radical polymerization of an ethylenically unsaturated anhydride in combination with other suitable olefinic monomers, a free radical initiator (e.g. benzoyl peroxide, 2,2xe2x80x2-azobisisobutyronitrile, 2,2xe2x80x2-azobis(methylisobutyronitrile), dimethyl 2,2xe2x80x2-azobisisobutyrate and lauroyl peroxide) and optionally a chain transfer agent (e.g. carbon tetrabromide). Tetrahydrofuran (THF) is the common solvent used as polymerization medium in the laboratory. A typical process used in the production of such polymers involves three steps: polymerizing monomers in tetrahydrofuran (THF) in the presence of a free radical initiator for 16-24 hours, isolating polymer from THF and removing unconverted monomers by precipitation in a large excess of a non-solvent such as hexane, drying the polymer in an oven for 14-36 hours under vacuum at a temperature less than 70xc2x0 C. and dissolving the dried polymer in a suitable solvent or solvent mixture typically employed in microelectronic industry. Typical examples of such solvents are propylene glycol methyl ether acetate (i.e., PGMEA), 2-heptanone and n-butyl acetate. The polymerization time is determined by residual amounts of monomers along with maleic anhydride, which can contribute to absorbance of photoresist films.
Examples of the various conventional methods for producing such anhydride-containing polymers for radiation sensitive compositions are set forth in U.S. Pat. Nos. 6,165,682 and 5,843,624, which are incorporated herein by reference.
Production of such polymers in larger scale quantities may be limited due to the use of THF as polymerization solvent. THF poses several safety hazards on a production scale due to low flash point (xe2x88x9217xc2x0 C.), high flammability and low boiling point (65-66xc2x0 C.). Hexane also poses safety hazards due to low boiling point (69xc2x0 C.) and high flammability. Since THF is not a suitable solvent for photoresist applications, polymers must be isolated and redissolved in appropriate solvents. Moreover, the isolation step generates large quantities of waste solvent. The third step of drying the polymer for 14-36 hours adds to the energy cost and cycle time. The last steps also introduce the potential for trace metals (microelectronic materials must contain no more than 10 ppb) and water contamination (water can open the maleic anhydride in the backbone which can in turn degrade the polymer). In addition, longer polymerization times of 16 to 24 hours further increase cost of production. Therefore, it is the objective of this invention to develop a novel process for producing radiation-sensitive maleic anhydride-containing polymers. The novel process is developed utilizing environmentally friendly solvents and is targeted for lower production cost.
Solvents such as alcohols are not available for ethylenically unsaturated anhydride polymerizations because of reactivity concerns. Common techniques for controlling molecular weight (MW) (e.g. dodecanthiol or alcohol change transfer agents) cannot be used because of reactivity. Thus, synthetic processes for anhydride based polymers have significant limitations that other synthetic processes do not have in the preparation of anhydride polymers with low trace metals, low water content, controlled molecular weight, and narrow dispersivity at reasonable cost.
Therefore, the present inventors have developed a novel synthetic process for producing anhydride-containing polymers for radiation-sensitive compositions that is environmentally friendly, of reasonable cost and prepares polymers with controlled MW, dispersivity, and trace metal contamination.
The present invention also provides many additional advantages which shall become apparent as described below.
The synthetic process of this invention for producing anhydride-containing polymers comprises:
a) providing a reaction mixture comprising: an ethylenically unsaturated anhydride monomer, at least one ethylenically unsaturated non-anhydride monomer, a free radical initiator, and an alkyl substituted THF solvent having the general structure of formula 1: 
where R1, R2, R3, and R4 are independently chosen from the group hydrogen and C1-C4 linear or branched alkyl with the proviso that at least one of R1, R2, R3, and R4 is not H;
b) polymerizing the reaction mixture; and
c) removing, such as by distillation, unreacted monomers, the alkyl substituted THF solvent, and any low-boiling volatile reaction products (i.e., reaction products having a lower boiling point than the boiling point of any co-solvent employed).